The invention relates in general to steam generators or boilers and more particularly to a feedwater heater and feedwater heating process for a heat recovery steam generator.
Natural gas represents a significant fuel to produce of electrical energy in the United States. It burns with few emissions, and is available throughout much of the country. Moreover, the plants which convert it into electrical energy are efficient and, in comparison to hydroelectric projects and coal-fired plants, they are relatively easy and inexpensive to construct. In the typical plant, the natural gas burns in a gas turbine, causing the rotor of the turbine to revolve and power an electrical generator to which the rotor is connected. Turbine exhaust gases—essentially air, carbon dioxide and steam—leave the gas turbine at about 1200° F. (649° C.) and are a significant source of energy. To harness this energy, the typical combined cycle, gas-fired, power plant also has a heat recovery steam generator (HRSG) through which the hot exhaust gases pass to produce steam which powers a steam turbine which, in turn, powers another electrical generator. The exhaust gases leave the HRSG at temperatures as low as 150° F. (66° C.).
The steam turbine and the HRSG operate within a loop that also contains a condenser and a feedwater pump. The steam generated by the HRSG passes through the turbine and then into the condenser where it is condensed back into liquid water. The pump delivers that water to the HRSG at about 100° F. (38° C.) or perhaps a lower temperature. The water enters the HRSG at a feedwater heater or economizer which elevates its temperature for subsequent conversion into steam within an evaporator and superheater that are also part of the HRSG.
Often the feedwater requires deaeration with a deaerator to remove dissolved gases from the feedwater to prevent corrosion of the system. Feedwater entering a deaerator needs to be approximately 20° F. below the deaerator operating temperature for proper operation. The temperatures shown in FIG. 1 are merely illustrative as the temperatures can vary depending on the application.
Generally, feedwater heaters have tubes produced from costly high alloy material to withstand the dissolved gases in feedwater, such as a high oxygen concentration. Therefore, it would be advantageous to remove the dissolved gases from the feedwater so that feedwater heater tubes can be produced using more economical materials, such as carbon steel.
The exhaust gas in an HRSG includes carbon dioxide and water in the vapor phase, but also includes traces of sulfur in the form of sulfur dioxide and trioxide. Those sulfur compounds, if combined with water, produce sulphuric acid which is highly corrosive. As long as the temperatures of the heating surfaces remain above the acid dew point temperature of the exhaust gas, SO2 and SO3 pass through the HRSG without harmful effects. But if any surface drops to a temperature below the acid dew point temperature, sulphuric acid will condense on that surface and corrode it.
Dew point temperatures vary depending on the fuel that is consumed. For natural gas, because of the sulphuric acid content the temperature of the heating surfaces should not fall below about 140° F.
Generally, an HRSG comprises a casing having an inlet and an outlet and a succession of heat exchangers-namely a superheater, an evaporator, and a feedwater heater arranged in that order within the casing between the inlet and outlet.
Such heat exchangers for an HRSG can have multiple banks of coils, the last of which in the direction of the gas flow can be a feedwater heater. Surfaces vulnerable to corrosion by sulphuric acid do exist on the feedwater heater. The feedwater heater receives condensate that is derived from low-pressure steam discharged by the steam turbine, and elevates the temperature of the water. Then the warmer water from the feedwater heater flows into one or more evaporators that convert it into saturated steam. That saturated steam flows on to the superheater which converts it into superheated steam. From the superheater, the superheated steam flows to the steam turbine.
In this process, by the time the hot gas reaches the feedwater heater at the back end of the HRSG, its temperature is quite low. However, that temperature should not be so low that acids condense on the heating surfaces of the feedwater heater.
Generally, in the above-discussed process, most HRSGs produce superheated steam at three pressure levels-low pressure (LP), intermediate pressure (IP) and high pressure (HP). An HRSG can thus have a superheater and also can have what are termed an LP Evaporator, an HP Economizer, and an IP Economizer. The feedwater heater typically discharges some of the heated feedwater directly into an LP evaporator.
A feedwater heater, or preheater, in a steam generator extracts heat from low temperature gases to increase the temperature of the incoming condensate before it goes off to the LP evaporator, HP economizer, or IP economizer. Multiple methods have been used to increase the temperature of the condensate before it enters any part of the preheater tubes within the gas path (e.g., recirculation pump, external heat exchanger). These methods are used to prevent the exhaust gas temperature from dropping below the acid dew point and causing sulphuric acid corrosion.
An overall illustration of a system which features use in a heat-recovery steam generator (HRSG) appears in U.S. Pat. No. 6,508,206 B1 (hereafter “'206 Patent”). FIG. 4 of the '206 Patent illustrates an arrangement with a superheater 18 located at the farthest position upstream. FIG. 4 of the present application generally refers to members such as a superheater and IP Economizer as upstream coils UC.
FIG. 3 of the present application shows a layout perspective similar to that shown in FIG. 3 of the '206 Patent. FIG. 3 of the present application discloses a gas turbine G that discharges hot exhaust gases into an “HRSG”, which extracts heat from the gases to produce steam to power a steam turbine S. The gas turbine G and steam turbine S power the generators E that are capable of producing electrical energy. The steam turbine S discharges steam at a low temperature and pressure into a condenser CN, where it is condensed into liquid water. The condenser CN is in flow connection with a condensate pump CP that directs the water back to the HRSG as feedwater. That water can pass through an external water-to-water heat exchanger EWTWEX that is located outside of the casing CS, and thus is external to the internal exhaust gas flow path through the HRSG duct.
The HRSG has a casing CS within which are heat exchangers. Hot gases, such as discharged from a gas turbine, enter the casing CS and pass through a duct having an inlet IN and an outlet OT. During such process, that gas passes through heat exchangers.
The casing CS of the HRSG generally will have a floor F, a roof R, and sidewalls that extend upwardly from the floor F to the roof R. The heat exchangers are positioned within the casing CS. The floor F and roof R extend between the sidewalls so that the floor F, sidewalls and roof R help to form the internal duct of the casing CS of the HRSG, through which the exhaust gas passes.
Generally, the heat exchangers comprise coils that have a multitude of tubes that usually are oriented vertically and arranged one after the other transversely across the interior of the casing CS. The coils are also arranged in rows located one after the other in the direction of the hot gas flow depicted by the arrows in FIGS. 3 and 4 of the present application. The tubes contain water in whatever phase its coils are designed to accommodate. The length of the tubes can be as great as 80’ tall.
Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings.